The present and foregoing application claim priority to Japanese Application No. P2000-182276 filed Jun. 16, 2000. All of the foregoing applications are incorporated herein by reference to the extent permitted by law.
1. Field of the Invention
The present invention relates to a gel electrolyte obtained by gelling nonaqueous electrolyte solution and more particularly to a nonaqueous electrolyte battery using the gel electrolyte.
2. Description of Related Art
In these days, batteries have industrially occupied important positions as the power sources of portable electronic devices. In order to realize the compact and light devices, it has been necessary to reduce the weight of the batteries and efficiently employ accommodation spaces in the devices. Lithium batteries high in their energy density and output density are most suitable for meeting the above described demand.
Although a battery high in degree of freedom for its configuration, a thin sheet type battery with a large area and a thin card type battery with a small area have been demanded among many batteries, it has been difficult to manufacture a thin battery with a large area by a conventional method for using a metallic can as an outer casing.
In order to solve this problem, batteries using gel electrolyte which employ organic or inorganic solid electrolyte or polymer gel have been studied. These batteries are characterized in that the electrolyte is stabilized, the thickness of the electrolyte is fixed and an adhesive strength exists between an electrode and the electrolyte to maintain a contact force. Therefore, it is not necessary to seal electrolyte solution by a metallic outer casing or to exert pressure on a battery element. As a result, a film type outer casing can be used to make a battery thin.
However, an all-solid electrolyte is low in its ionic conductivity so that it is difficult to actually utilize it for manufacturing a battery. Accordingly, the gel electrolyte is currently considered to be important.
As the outer casing, a multilayer film composed of a polymer film or a metallic thin film or the like may be employed. Specially, a moisture-proof multilayer film composed of a heat sealing resin layer and a metallic foil layer is desirable as a candidate of an outer casing material, because the moisture-proof multilayer film can easily realize a sealed structure by a hot seal, is excellent in strength airtightness of the multilayer film itself and lighter, thinner and more inexpensive than the metallic outer casing.
As matrix polymers used for the gel electrolyte, there are known polyether compounds such as polyethylene oxide, polyacrylonitrile (PAN), polyvinylidene fluoride (PVdF), polymethyl methacrylate, or the like. Especially, since the gel electrolyte can be simply manufactured by using the mixture of the PVdF and a nonaqueous solvent and electrolyte, heating and cooling the mixture, dissolving the mixture in a diluent solvent and then volatilizing the solvent, and further, the PVdF is excellent in its electrochemical stability, the PVdF is preferable as the matrix polymer.
On the other hand, as the nonaqueous solvents, ethylene carbonate (EC) and propylene carbonate (PC) are electrochemically stable, high in their dielectric constant, and useful. In this case, the EC has a melting point as high as 38xc2x0 C. Further, the PC has a viscosity as high as 2.5 mPas, so that the ionic conductivity at low temperature is disadvantageously deteriorated.
Thus, for the purpose of solving the above described problem, solvents with low viscosity are used in an ordinary lithium-ion battery. As the solvents with low viscosity, straight chain carbonates such as dimethyl carbonate (DMC), ethylmethyl carbonate (EMC) and diethyl carbonate (DEC), or the like are widely employed, because they are electrochemically stable. The viscosity of the solvents with low viscosity is as low as 0.6 mPas to 0.8 mPas. The melting point of the dimethyl carbonate (DMC) is 3xc2x0 C., that of the ethylmethyl carbonate (EMC) is xe2x88x9255xc2x0 C., and that of the diethyl carbonate (DEC) is xe2x88x9243xc2x0 C. Accordingly, these carbonates are used for the gel electrolyte at the same time, so that it is anticipated that the ionic conductivity of the gel electrolyte can be improved at low temperature.
However, when the PVdF is used as the matrix polymer, in case these solvents with low viscosity are employed together, a serious problem will be generated in view of gelling. Specifically, although the polyvinylidene fluoride (PVdF) has a compatibility to some degree with the solvents having the high dielectric constant such as the EC, PC, or the like, it has a low compatibility with the solvents of low viscosity such as the DMC, EMC, DEC, or the like. Therefore, when these solvents of low viscosity are employed together with the PVdF, a stable gel electrolyte cannot be undesirably formed.
Accordingly, in the gel electrolyte using the PVdF as the matrix polymer, it is difficult to use the solvents of low viscosity together with the PVdF. Consequently, the ionic conductivity of the gel electrolyte at low temperature is inconveniently inferior to that of electrolyte solution.
The present invention was devised by taking the above described problems into consideration and it is an object of the present invention to provide a gel electrolyte good in its ionic conductivity at low temperature even when polyvinylidene fluoride (PVdF) is used as a matrix polymer and further provide a nonaqueous electrolyte battery excellent in its battery property at low temperature.
The inventors of the present invention carried on various studies and examinations for a long period, and after that, they found that a partly carboxylic acid modified copolymer obtained by copolymerizing, for instance, maleic acid with vinylidene fluoride is excellent in compatibility with a straight chain carbonate, chemical stability, strength of gel and a liquid retaining property and useful for the matrix polymer of the gel electrolyte.
The present invention is completed in accordance with the above mentioned knowledge. According to one aspect of the present invention, there is provided a gel electrolyte in which nonaqueous electrolyte solution obtained by dissolving electrolyte salt containing Li in a nonaqueous solvent is gelled by a matrix polymer including a copolymer as a main component which contains vinylidene fluoride as a monomer unit, wherein at least one kind of structure selected from structures formed by esterifying a part or all of a carboxyl group or a carboxylic acid, or an acetic anhydride structure is introduced into the copolymer.
Further, according to another aspect of the present invention, there is provided a nonaqueous electrolyte battery comprising: an anode and an anode and a gel electrolyte in which nonaqueous electrolyte solution obtained by dissolving electrolyte salt containing Li in a nonaqueous solvent is gelled by a matrix polymer including a copolymer as a main component which contains vinylidene fluoride as a monomer unit, wherein at least one kind of structure selected from structures formed by esterifying a part or all of a carboxyl group or a carboxylic acid, or an acetic anhydride structure is introduced into the copolymer.
Since the polyvinylidene fluoride (PVdF) is low in its compatibility with a solvent of low viscosity, a stable gel electrolyte is hardly manufactured when the PVdF is employed as it is. On the contrary, carboxylic acid modified PVdF (at least one kind of structure selected from a structure obtained by esterifying a part or all of carboxyl group or carboxylic acid, or acetic anhydride is introduced) can dissolve a solvent of low viscosity having a low boiling point therein and retain the liquid thereof. Therefore, the carboxylic acid modified PVdF (polyvinylidene fluoride) is employed as the matrix polymer, so that the ionic conductivity of the gel electrolyte is improved at low temperature.
Further, in the nonaqueous electrolyte battery using the above described gel electrolyte, the improvements of a current characteristic and a low temperature characteristic can be realized while the advantages of a polymer battery such as no leakage of liquid, a light and compact form or the like are maintained, since an outer casing can be made of a laminate film.